A CMOS image sensor consists of an array of Field Effect transistors, each of which acts as a pixel, together with various support circuits for driving and reading out the signal. A bad pixel is defined as a pixel whose response is noticeably different from the response of the other pixels in the array under dark or uniformly illuminated conditions. Pixel defects can be caused for many reasons including high leakage, circuit defects, dust particles, scratches, color filter nonuniformity, or microlens defects. The two extremes for such defects are: dead pixels, which are always dark, and hot pixels which are always saturated. No matter their origin, bad pixels degrade image quality because even one defective pixel can stand out in an image made up of millions of good pixels.
Color interpolation, sharpening and other software-based image processing operations are often used to neutralize the effects of bad pixels but these can actually further degrade image quality by corrupting good pixels that are the neighbors of a bad pixel. Additionally, a bad pixel introduces high frequency components to an image which impacts the compression ratio.
Another method of dealing with defective pixels involves the use of two exposures in succession with a small diagonal shift of the entire sensor of 10 to 20 pixels between the two exposures. The computer then combines the two exposures into a single image which will have very few missing pixels. For further accuracy, this method can be extended to three successive exposures with diagonal shifts of the sensor between each exposure.
Although it is highly desirable to have an image sensor that is entirely defect-free, selecting arrays having only good sensors with not a single bad pixel is not a viable alternative as it would drive down manufacturing yield and significantly increase cost. The present invention teaches how image sensors may be made to be effectively defect-free by detecting, and then correcting for, the bad pixels. An approach of this type carries with it the additional benefit of making arrays, that might otherwise have been rejected after testing, useable.
A routine search of the prior art was performed but no references that teach the exact processes and structures of the present invention were discovered. Several references of interest were, however, encountered along the way. For example, in U.S. Pat. No. 5,528,043 and U.S. Pat. No. 5,886,353, Spivey, et al. both describe an imaging system for producing images from electromagnetic radiation such as x-rays. Their system includes a detector comprised of a radiation-absorbing layer sandwiched between an array of CMOS integrated circuits (pixel circuits) and a surface electrode layer transparent to the radiation. Each of the pixel circuits in the array has a charge-collecting electrode. An external voltage applied between the surface electrode layer and the charge collecting electrodes produces an electric field across the thickness of the absorbing layer pixels and even missing rows or columns are corrected by having the computer assign values to the missing pixels by interpolation between the values of the neighboring pixels.